outlook classic running very slow

If Outlook Classic is running very slow, you can try the following steps to improve its performance:

1. Install Latest Updates: Ensure that you have the latest updates for Outlook installed. This can often resolve performance issues.
2. Run Diagnostics: Use the Classic Outlook Advanced Diagnostics troubleshooter to create a detailed configuration report. This tool can help identify issues affecting performance.
   • Note: The troubleshooter does not work in the new Outlook for Windows, and it should be run on the same Windows device where classic Outlook is installed.
3. Disable Add-ins: Sometimes, previously installed add-ins can interfere with Outlook's performance. Consider disabling any unnecessary add-ins.
4. Reduce Mailbox Size: Large mailboxes can slow down performance. Try archiving old emails or deleting unnecessary items to reduce the size of your mailbox.
5. Close Unused Shared Folders: If you have many shared folders or mailboxes open, consider closing those that you do not need, as they can impact performance.
6. Check Antivirus Software: Ensure that your antivirus software is up to date, as outdated software can conflict with Outlook.
7. Repair Outlook: If the problem persists, consider repairing your Microsoft 365 programs to fix any potential issues.

By following these steps, you should be able to improve the performance of Outlook Classic.

Hi @Matthew Thomlinson

Thank you for posting your question in the Q&A Forum. 

We noticed that Outlook Classic can sometimes run slowly, especially when moving emails or syncing with the server. To help improve performance and make your experience smoother, please follow the steps below: 

Start Outlook in Safe Mode 

• Press Windows Key + R, type outlook.exe /safe, and press Enter.  
• If performance improves, disable add-ins via File > Options > Add-ins > COM Add-ins > Go…

Reduce Mailbox Size 

• Go to File > Info > Tools (Mailbox Settings) > Mailbox Cleanup. 
• Use this tool to identify and Archive older emails from folders with a very large number of items (e.g., Sent Items or Inbox). 

Rebuild the Outlook Data File (.OST) 

• Close Outlook.  
• Navigate to %localappdata%\Microsoft\Outlook
• Locate the .ost file for your account and rename it (e.g., add .old). 
• Restart Outlook. A new data file will be created and synchronized automatically. 

Create a New Outlook Profile 

• Open Control Panel > Mail (Microsoft Outlook) > Show Profiles… > Add...
• Add your account to the new profile. 
• Select Prompt for a profile to be used and restart Outlook to test synchronization. 

For a more comprehensive checklist of possible causes (large OST/PST files, add-ins, indexing, antivirus integration, etc.), please review this Microsoft support article: How to troubleshoot performance issues in Outlook - Microsoft Support 

Following these steps usually resolves the most common performance issues in Outlook Classic. If Outlook is still running slowly after trying this, please let us know so we can provide further assistance. 

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